This invention is concerned with the purification of 2,2-bis(4-hydroxyphenyl) propane (hereinafter identified as "bisphenol-A" or "BPA"). More particularly, the invention is directed to a method for recovering bisphenol-A in a purified state from crude bisphenol-A.
Crude bisphenol-A is the isolated product of commercial processes for preparing bisphenol-A. It is a mixture of bisphenol-A and impurities derived from the BPA synthesis reaction. An example of BPA synthesis reaction is the acid-catalyzed condensation of phenol and acetone where phenol and acetone react in the presence of an acidic material such as sulfuric acid, hydrochloric acid, cation exchange resin, etc.
The crude bisphenol-A produced contains undesirable impurities such as the 2-(4-hydroxyphenyl)-2-(2-hydroxyphenyl) propane (hereinafter identified as "o,p-isomer") having the formula ##STR1## as well as other impurities including phenol itself used in making the bisphenol-A, a trishydroxyphenyl compound of the formula ##STR2## (hereinafter identified as "BPX-1"), small amounts of other impurities such as the two compounds having the formulas ##STR3## (herein identified as "LD/CD"), and some impurities with unknown structure (herein identified as "color bodies").
Since bisphenol-A is used in making polycarbonate resins by reaction of the latter with either phosgene or diphenyl carbonate, or for making epoxy resins, both resins being used extensively in commercial applications involving molding, casting, and sheet forming purposes, it is highly important that the monomeric bisphenol-A used to make such resins be as pure as possible in order to avoid adverse effects on the properties of the polymers thus obtained.
The preparation of the bisphenol-A by the acid-catalyzed reaction of phenol and acetone is usually carried out in excess phenol (&gt;2 moles per mole acetone). This reaction mixture is subjected to a series of distillation steps to recover crude bisphenol-A which is the starting point for making bisphenol-A of high purity.
The conventional way of recovering a pure bisphenol-A product from crude bisphenol-A involves crystallization in presence of an organic solvent as described in British Pat. Nos. 795,236 and 975,863 and German Pat. No. 1,153,763. According to these patents, crude molten bisphenol-A is first dissolved in a suitable organic solvent. The solution is then cooled to yield pure bisphenol-A crystals, which are recovered by filtration. However, this process suffers in that the crystals produced are fine, powdery and needle-like which are difficult to handle, store and dry. These crystals also permit the organic solvent to occlude during crystallization, which can not be removed readily during the drying step and hence the organic solvent ends up in the downstream polymerization process.
A method which uses water as the crystallization medium for bisphenol-A is described in U.S. Pat. No. 3,326,986. According to this patent, the isolated crude bisphenol-A in molten form is mixed with water and the mixture is cooled to yield large less needle-like crystals of bisphenol-A. Separation of these crystals from the mother liquor followed by an organic solvent wash results in purified bisphenol-A. Although the process described within the above referenced patent avoids occlusion by an organic solvent during the crystal formation step and yields large, less needle-like crystals, the purification is limited, which appears to be due to limited washing of the crystals on the solid/liquid separation equipment after the mother liquor is removed. Theoretically, it should be possible to provide a very effective washing of the crystals obtained from the water crystallization process by repeating the steps of crystal separation, reslurrying the crystals with organic solvent and then crystal separation. However, such a process scheme is not viable since several complex operating steps and high costs are involved.
The present invention demonstrates a simplified method for obtaining high purity bisphenol-A from crude bisphenol-A utilizing an aqueous crystallization process followed by a treatment with an organic washing solvent which is more effective in removing surface impurities than techniques used previously.